In order to reduce fuel consumption, as well as pollutant emissions of vehicles equipped with combustion engines and known from experience, the combustion engine for different vehicle concepts is shut off during certain operating states. These functions are also known as motor start/stop functions, which are activated or de-activated, depending on the operating state of various vehicle components, and cause the combustion engine to be shut off if the vehicle is standing still even for short periods.
In order to avoid impairment of a conventional drive operation by a motor start/stop function, a short starting procedure of the combustion engine and immediate traction buildup are needed within the drive mechanism when the driver causes the vehicle to continue, particularly when entering heavily traveled streets with the right of way. In conventional automatic transmissions or automated gearboxes, which are built with shifting elements that are designed with a wet multiple, disk clutch or brake, the shifting elements are mainly supplied with the necessary control pressure, via a transmission pump, only when the combustion engine is running.
The hydraulically operated shifting elements usually feature at least one piston element, such that one multi-plate assembly of a shifting element is more or less strongly pressed, depending on a hydraulic traction pressure, and via which a preferably operationally-dependent, demanded transmission capacity of a shifting element can be set. The piston elements of the shifting elements are always tensioned against an operating pressure inside the piston space in the direction of a shifting element by way of a spring device, whereby the piston elements are mainly acted on by the total force components that are the result of the operating pressures, as well as the spring devices that move the piston elements toward the opening of the shifting element, or a first end position of the piston elements, or in the locking direction of the shifting elements, or a second end position of the piston elements, and also maintain the preset shifting positions between the two end positions.
If the operating pressure in the piston spaces of the shifting elements of a drive mechanism drops when the combustion engine is shut off, due to the subsequent lacking of supply from the main transmission pump, the piston elements will be moved by the individually associated spring devices to their first end position, whereby the hydraulic fluid volume present inside the piston spaces, is basically fully forced out over time.
When the combustion engine is again operated and traction is built up in the transmission, the piston elements of the shifting element that must be activated must be moved in the direction of their second end position and the multi-plate assemblies must be activated with the operating forces corresponding to the traction forces that are present within the drive mechanism. For this an air gap of the shifting elements to be engaged must, first, be overcome and the shifting elements to be engaged must then be completely engaged by increasing the control pressure according to the preset shifting characteristics. The compensation of the air gap of the shifting elements, as well as their inclusion in the power flow of a drive mechanism, is achieved by supplying a certain hydraulic fluid volume to a piston space of a hydraulically controlled, shifting element that must be engaged, which must be supplied by the transmission pump that is driven by the started combustion engine.
Before the vehicle startup, when the combustion engine has been switched off, if several shifting elements of a transmission have been disengaged by the above spring devices and have to be engaged before the vehicle moves on, the time between the start of the combustion engine starting procedure and the time when traction is fully restored within the transmission may possibly be extended such that a vehicle equipped with a motor start/stop function cannot be operated to the intended extent within the period that a driver would desire or expect, since the possibly fully depleted shifting elements must be supplied by the main transmission pump that is driven by the combustion engine with a substantial hydraulic fluid volume until the traction in the drive mechanism is restored.
In order that vehicles with conventionally designed transmission mechanisms with implemented motor start/stop function can be operated in the desired way and manner within the time desired or expected by a driver, known vehicles have, in addition to the main transmission pump, another hydraulic pump that is preferably operated by an electric motor whose supply volume is independent of the rotational speed of the combustion engine and, when the pressure supply is not available from the main transmission pump in the hydraulic system of the transmission mechanism, it generates at least a pressure which is used to prevent a spring-generated depletion of the shifting elements that must be engaged to generate the traction within the transmission.
Disadvantageously, the above design of a drive mechanism with a main transmission pump driven by a combustion engine and an auxiliary pump driven by an electric motor, when compared to a drive mechanism designed without an auxiliary pump, leads to a reduction in the efficiency of the transmission, as well as to an increase in the manufacturing costs of the gear mechanism. The electrical power requirement of the active auxiliary pump also creates an undesirable demand on the on-board, power circuit. Also, the positioning of the auxiliary pump, preferably located outside of the housing of the drive mechanism, undesirably increases the installation space requirement within the vehicle, as well as the installation expense owing to the hydraulic connection of the auxiliary pump to the hydraulic system of the drive mechanism and the electrical connection of the auxiliary pump to the electrical control and regulation system of the vehicular drive train.
The alternative to this solution using pressure reservoirs are known from experience, which are used to store pressurized hydraulic fluid and then supply this fluid, as needed, to the hydraulic system of a drive mechanism to control the clutches that are to be engaged.
However, such pressure reservoir solutions have the disadvantage that a significant expense is required to store hydraulic fluid at the desired pressure within the reservoir. Main pump filling of the pressure reservoir also increases the pump input momentum of the main transmission pump to an undesirable extent, which likewise negatively affects the overall efficiency of a drive mechanism.
It is, therefore, the object of the present invention to provide a device for operating a shifting element of a drive mechanism, such that traction can be made available in a drive mechanism within shorter operating times in comparison with the drive mechanisms known from experience, simultaneously with good drive mechanism efficiency.